This invention relates to a so-called microturbine power plant. More particularly, this invention relates to a combination of a microturbine and a secondary fuel burner in the gas turbine exhaust gas. The gas turbine drives an electrical generator and can have the generator unit built directly onto the turbine shaft. The secondary fuel burner is used to improve the quantity and quality of the heat content of the exhaust gas to a point where it can be used as a heat source for a number of purposes, such as for an absorption chiller, for space heating, for steam generation, and for water heating. As a consequence of the properties of the secondary fuel burner this invention provides a means to obtain useful heat recovery without any increase in NOx formation.
In the past, electricity for commercial and domestic use has been generated on a large scale. Originally, either hydroelectric generation or coal fired power stations were used; in more modern practice oil, natural gas, and atomic energy are all used as alternatives to coal to provide the required heat. In recent times, a need has been identified for a small power plant to provide the electrical needs of a small facility, typically having a power requirement in the range of 25kW to 500kW. For a power plant of this size, both atomic energy and coal are not suitable as fuels. Small generating systems have been developed having this power rating in which the prime mover is a gas turbine, in which a liquid or gaseous hydrocarbon fuel is burnt to generate the required heat. A typical microturbine unit of this type is described by Gjerde, in U.S. Pat. No. 3,457,902. As described, the microturbine combustor utilises only a small proportion of the oxygen in the intake air to burn the hydrocarbon fuel; the microturbine exhaust gas typically contains about 16-19% oxygen by volume, compared to about 21% in the intake air. In order to minimise fuel use, the turbine exhaust gas is used to heat the intake air in a recuperator; such units are known as recuperated microturbines.
In practise, it has been found that recuperated microturbine units have three disadvantages.
First, although the exhaust gas leaving the recuperator is hot, it does not have enough heat content to be very useful beyond being used to generate hot water. Second, although the combustion conditions can be improved to use more of the oxygen to obtain higher exhaust gas temperatures, this will also cause increased NOx formation. Recuperated turbines generally operate at a low rate of NOx emission: values as low as 0.06 g/kWh or 1.8 ppm NOx are known. This combination of operating parameters provides exhaust gasses leaving the recuperator at a temperature of no more than 250xc2x0 C., which is only useful for space heating and hot water.
Third, in addition to Gjerde noted above, several proposals have been made to burn more fuel in at least a proportion of the exhaust gas, for instance by Freeman et al. in U.S. Pat. No. 3,934,553 and by Vitterick, in U.S. Pat. No. 5,461,853. These proposals rely on the oxygen consumption in the turbine combustor. Two difficulties then arise. First, the quantity of exhaust gas available from the turbine is only constant if the working load on the turbine is also constant: in practise this is not often the case. Second, if the added fuel is to be burnt efficiently the production of NOx increases significantly.
There is still, therefore, a need for a recuperated microturbine unit which includes an afterburner system which will accommodate quite significant differences in the amount of available exhaust gas, which will increase the heat content of the exhaust gas, and which will burn the added fuel at a low combustion temperature without any significant increase in NOx formation.
This invention seeks to provide a recuperated microturbine system including a duct burner in the exhaust gas which accommodates significant variations in the exhaust gas flow from the recuperator, which burns the added hydrocarbon fuel, for example natural gas, reasonably efficiently, and which does not utilise combustion conditions which generate additional NOx. In the low temperature duct burner of this invention, a wire mesh burner is used to burn the added fuel in part of the exhaust gas from the recuperator. This burner is operated in either a more or less radiant combustion mode or preferably in the so-called xe2x80x9cblue flamexe2x80x9d mode, with lean gas mixtures at 50% of the stoichiometric fuel to gas ratio. The remainder of the recuperator exhaust gas, which bypasses the afterburner, is heated by heat transfer from the wire mesh burner. The fuel flow, and exhaust air flow, to the duct burner are both controlled to provide stable combustion, in relation to the heat content required in the exhaust gas.
Thus in a first broad embodiment this invention provides a duct burner, for use in an exhaust gas stream contained within an exhaust gas duct from a recuperated microturbine, including in combination:
(a) an exhaust gas intake tube, of the same cross sectional shape as, and smaller cross sectional area than, the exhaust gas duct, mounted substantially coaxially with the exhaust gas duct, which intake tube has a first upstream end which receives exhaust gas and a second downstream end;
(b) a throttle means located in the intake tube adjacent its first upstream end constructed and arranged to control exhaust gas flow in the intake tube;
(c) a fuel feed for a hydrocarbon fuel located in the intake tube adjacent to the throttle plate; and
(d) a wire mesh burner attached to the second downstream end of the intake tube and extending in a down stream direction from the second end of the intake tube;
wherein the wire mesh burner comprises a burner membrane consisting of a non-sintered fabric type membrane fabricated from heat resistant stainless steel fibre bundles in which the fibres have a substantially parallel arrangement and an equivalent fibre diameter of from about 1xcexc to about 150xcexc.
Alternatively, the duct burner further includes:
(e) a perforated radiation tube, of the same cross section shape as, of a larger cross sectional area than, and mounted substantially coaxially with, the intake tube to extend in a down stream direction from the second end of the intake tube to a point beyond the end of the wire mesh burner.
Preferably, the wire mesh burner membrane has the same cross sectional shape as the intake duct. More preferably, the wire mesh burner is conical, with its wide end attached to the second end of the intake duct.
Preferably, the hydrocarbon fuel is natural gas.
Preferably, both the exhaust gas duct, the exhaust gas intake tube and the perforated radiation tube if used are cylindrical, and the wire mesh burner is conical, with its wide end attached to the second end of the intake duct. More preferably, both the exhaust gas duct, the exhaust gas intake tube and the perforated radiation tube if used are cylindrical, the radius of the exhaust gas intake tube is about one half of the radius of the exhaust gas duct, and the wire mesh burner is conical, with its wide end attached to the second end of the intake duct.
Alternatively, both the exhaust gas duct, the exhaust gas intake tube and the perforated radiation tube if used have a rectangular cross section, the cross sectional area of the exhaust gas intake tube is about one quarter of the cross sectional area of the exhaust gas duct, and the wire mesh burner has a pyramidal shape, with its wide end attached to the second end of the intake duct.
In a further alternative, both the exhaust gas duct, the exhaust gas intake tube and the perforated radiation tube if used have a square cross section, the cross sectional area of the exhaust gas intake tube is about one quarter of the cross sectional area of the exhaust gas duct, and the wire mesh burner has a square pyramidal shape, with its wide end attached to the second end of the intake duct.
Preferably, sufficient hydrocarbon fuel is burnt in the wire mesh burner to increase the temperature of the incoming exhaust gas by at least about 50xc2x0 C. and by no more than about 500xc2x0 C.
Preferably, the throttle means comprises a plate rotatable on a axis which is substantially perpendicular to the axis of the exhaust gas intake tube.